This invention concerns the art of catalyst regeneration. More specifically, the present invention concerns a method for regenerating coke-contaminated particulate catalyst and avoiding nitrogen oxides contamination of flue gas formed by coke combustion.
Catalytic cracking systems employ catalyst in a moving bed or a fluidized bed. Catalytic cracking is carried out in the absence of externally supplied molecular hydrogen, and is, for that reason, distinctly different from hydrocracking, in which molecular hydrogen is added during the cracking operation. In catalytic cracking, an inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In a fluidized catalytic cracking (FCC) system, hydrocarbon feed is contacted with catalyst particles in a hydrocarbon cracking zone, or reactor, at a temperature of about 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The reactions of hydrocarbons at the elevated operating temperature result in deposition of carbonaceous coke on the catalyst particles. The resulting fluid products are separated from the coke-deactivated, spent catalyst and are withdrawn from the reactor. The coked catalyst particles are stripped of volatiles, usually by means of steam, and passed to the catalyst regeneration zone. In the catalyst regenerator, the spent catalyst is contacted with a predetermined amount of molecular oxygen. A desired portion of the coke is burned off the catalyst, restoring catalyst activity and simultaneously heating the catalyst to, e.g., 540.degree. C.-815.degree. C., usually 590.degree. C.-730.degree. C. Flue gas formed by combustion of coke in the catalyst regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Most fluidized catalytic cracking systems now use zeolite-containing catalyst having high activity and selectivity. Zeolite-type catalyst have a particularly high activity when the concentration of coke on the catalyst after regeneration is relatively low. It is therefore desirable to regenerate zeolite-containing catalysts to as low a residual carbon level as is possible, so as to obtain a relatively high activity and selectivity. It is also desirable to burn carbon monoxide as completely as possible during catalyst regeneration to obtain additional heat, especially when the concentration of coke on the spent catalyst is already quite low as a result of high catalyst selectivity. Among the ways suggested to help reduce the amount of coke on regenerated catalyst and to burn carbon monoxide to provide process heat, is enhanced carbon monoxide combustion in a dense-phase fluidized catalyst bed in the catalyst regenerator promoted by an active, combustion promoting metal. Metals have been used either as an integral component of the cracking catalyst particles or as a component of a separate particulate additive, in which the active metal is associated with a support other than the catalyst particles. Additive particles are mixed with catalyst particles in the circulating particulate solids inventory. Various ways of employing carbon monoxide combustion promoting metals in cracking systems have been suggested. In U.S. Pat. No. 2,647,860, it is proposed to add 0.1-1 weight percent chromic oxide to a cracking catalyst to promote combustion of carbon monoxide to carbon dioxide and to prevent after burning. In U.S. Pat. No. 3,808,121, it is proposed to introduce relatively large sized particles containing a carbon monoxide combustion promoting metal into a cracking catalyst regenerator. The circulating particulate solids inventory, comprised of relatively small-sized catalyst particles, is cycled between the cracking reactor and the catalyst regenerator, while the combustion promoting particles remain in the regenerator because of their size. Oxidation promoting metals such as cobalt, copper, nickel, manganese, copper-chromite, etc., impregnated on an inorganic oxide such as alumina, are disclosed. Belgium Patent Application No. 820,181 suggests using catalyst particles containing platinum, palladium, iridium, rhodium, osmium, ruthenium or rhenium to promote carbon monoxide oxidation in a catalyst regenerator. An amount of the metal between a trace and 100 parts per million is added to the catalyst particle, either during catalyst manufacture or during the cracking operation, as by addition of a compound of the combustion promoting metal to the hydrocarbon feed. Addition of the promoting metal to the cracking system is said by the publication to decrease product selectivity in the cracking step by substantially increasing coke and hydrogen formation. Catalyst particles containing the promoter metal can be used alone or can be circulated in physical mixture with catalyst particles free of the combustion promoting metal. U.S. Pat. Nos. 4,072,600 and 4,093,535 disclose the use of combustion promoting metals in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
One problem encountered in some cracking operations using metal promoted combustion-type regeneration to provide substantially coke-free regenerated catalyst has been the generation of undesirable nitrogen oxides (NO.sub.x) in the flue gas formed during coke combustion. The present invention is directed, in part, toward providing a catalyst regeneration system, which accomplishes substantially complete removal of coke during catalyst regeneration, while substantially lowering the concentration of nitrogen oxide present in flue gas produced by the regeneration operation.
Representative of catalyst regeneration patent literature previously published are the following patents: U.S. Pat. No. 3,909,392 describes a scheme for enhancing carbon monoxide combustion by thermal means. Catalyst is used a heat sink for the increased heat production. U.S. Pat. No. 4,093,535 describes a scheme for enhancing carbon monoxide combustion by means of a noble metal combustion promoter. British patent publication No. 2,001,545 describes a two-stage system for regenerating catalyst, with partial catalyst regeneration being carried out in the first stage and secondary, more complete regeneration carried out in the second stage with a separate regeneration gas. U.S. Pat. No. 3,767,566 describes a two-stage regeneration scheme in which partial regeneration takes place in an entrained catalyst bed, and secondary, more complete regeneration takes place in a dense fluidized catalyst bed. A somewhat similar regeneration operation is described in U.S. Pat. No. 3,902,990, which discusses the use of several stages of regeneration, with dilute and dense-phase beds of catalysts being employed, and with the use of plural streams of regeneration gas. U.S. Pat. No. 3,926,843 describes a plural-stage regeneration scheme in which dilute phase and dense-phase coke burning are performed. British patent publication No. 1,499,682 discloses use of a combustion-promoting metal for enhancing carbon monoxide combustion. None of the above cited patents provides a method for forming a flue gas having low concentrations of both carbon monoxide and nitrogen oxides, while accomplishing essentially complete removal of coke from the catalyst.
Addition of a fuel such as "torch oil" to a cracking catalyst regenerator has been practiced commercially for the purpose of maintaining a cracking system in heat balance.